In muscle movement, the synergist is the muscle or group of muscles that stabilizes a joint around which the movement is occurring. The fixator is the muscle or group of muscles that stabilizes the origin of the primary muscle movement and the joint from which it originates, says PT Direct.
Muscles that are directly involved in producing a joint movement are called agonists. Agonist muscles are capable of increasing torque in the direction of the limb's movement. Movements can have more than one agonist, although there is often a single muscle acting as the prime mover because of the size of the muscle. Any muscle that indirectly assists in providing joint movement is the agonist's synergist.
The terms to describe these synergistic muscles are stabilizer, neutralizer and fixator. Fixators are active at one joint and during one movement. The fixator acts to eliminate the unwanted movement of the agonist, according to Ground Up Strength. The other role the skeletal muscle performs in a movement is that of antagonist. The antagonist in a movement is the muscle or muscles that oppose the agonist. When the agonist contracts and causes a movement to occur, the antagonist relaxes so as not to impede the movement, as explained by PT Direct.